3022

Downdraft Gasifier Engine SystemsSERIISP-271-3022 DE88001135 March 1988 UC Category.' 245

Handbook of Biomass

This handbook has been prepared by the Solar Energy Research Institute under the U.S. Department of Energy Solar Technical Information Program. It is intended as a guide to the design, testing, operation, and manufacture of small-scale [less than 200 kW (270 hpJ] gasifiers. A great deal of the information will be useful for all levels of biomass gasification. The handbook is meant to be a practical guide to gasifier systems, and a minimum amount of space is devoted to questions of more theoretical interest. We apologize in advance for mixing English and Scientifique Internationale (SI) units. Whenever possible, we have used SI units, with the corresponding English units fol lowing in parentheses. Unfortunately, many of the figures use English units, and it would have been too difficult to convert all of these figures to both units. We have sup plied a conversion chart in the Appendix to make these conversions easier for the reader.Mr. Bill Nostrand, one of our very helpful reviewers, died in May 1985. Bill was num

ber one in the ranks of those who became interested in gasification because of its poten tial for supplying clean, renewable energy. We all will miss him. The improvement of gasification systems will be noticeably slowed by his death.

We dedicate this book to the Bill Nostrands of this world who will bring gasifier systems to the level of safety, cleanliness, and reliability required to realize their full potential. Thanks, Bill. T_ B. Reed and A. Das Golden, Colorado

Solar Energy Research Institute

A Product of the

Solar Technical Information Program1 617 Cole Boulevard, Golden, Colorado 80401-3393

A Division of Midwest Resea rch InstituteOperated for the

U.S. Department of Energy

Acknowledgments

Since it is impossible for one or two authors to realistically comprehend a subject from all viewpoints, we have solicited input from leading workers in the field. Early versions were sent to a number of investigators, and each was invited to comment on and supplement our effort. We therefore express our heartfelt thanks to the following reviewers for greatly enhancing the quality of the final product: Dr. Thomas Milne, Solar Energy Research Institute Dr. Bjorn Kjellstrom, The Beijer Institute, Sweden Dr. Thomas McGowan, Georgia Institute of Technology Dr. Hubert Stassen, Twente University, The Netherlands Prof. Ibarra Cruz, University of Manila, The Philippines Mr. Matthew Mendis, World Bank Mr. Bill Nostrand, New England Gasification Associates We take final responsibility for the contents and omissions, and extend our apologies to those workers whose work we may have unknowingly omitted.

Organization and UseA gasifier converts solid fuel to gaseous fuel. A gasifier system includes the gasification reactor itself, along with the auxiliary equipment necessary to handle the solids, gases, and effluents going into or coming from the gasifier. The figure below shows the major components of a gasifier system and the chapters in which they are discussed.

Fuel Ch.3

Gasifier Ch.4, 5, 6

Gas measurement and cleaning Ch. 7, 8 Engine (or com bustor) Ch. 1 1

Whole ... .. .._____________ ..

system Ch. 9, 1 0

__________

--,l.

NoticeThis report was prepared as an account of work sponsored by an agency of the United States government. Neither the United States govern ment nor any agency thereof. nor any of their employees, makes any warranties, express or implied. or assumes any legal liability or respon sibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific"commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States govern ment or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States government or any agency thereof. Printed in the United States of America Available from: Superintendent of Documents U.S. Government Printing Office Washington, DC 20402 National Technical Information Service U.S. Department of Commerce 5285 Port Royal Road Springfield, VA 22161 Price: Microfiche A01 Printed Copy A07 Codes are used for pricing all publications, The code is determined by the number of pages in the publication, Information pertaining to the pricing codes can be found in the current issue of the following publications which are generally available in most libraries: Energy Research Abstmcts (ERA); Government Reports Announcements and Index (GRA and I) Scientific and Technical Abstmct Reports (STAR); and publica tion NTIS-PR-360 available from NTIS at the above address.

Contents

1 .0 Introduction and Guide to the Literature and Research 1.1 Role of Gasification in Biomass Conversion 1.2 Biomass Energy Potential . . . . 1.3 Guide to Gasification Literature 1.3.1 Bibliographies . . . . . 1.3.2 Books . . . . . . . . . . 1.3.3 Gasification Proceedings 1.3.4 Commercial Information 1.3.5 Producer Gas Research 1.3.6 Producer Gas R&D Funding 1.3.7 Federal Emergency Management Agency (FEMA) Gasifier Work 2.0 H istory, Current Developments, and Future Directions

2.1 Historical Development . . . . . . . . . . . 2.1.1 Early Development of Gasification 2.1.2 Vehicle Gasifiers . . . . 2.2 Current Development Activities 2.3 Future Development Directions

6 6 6 6 7 7

9 9 9 9

1 1 2 3 3 3 3 4 4 4 4

3.0 Gasifier Fuels 3.1 Introduction . . . . . . 3.2 Biomass Fuel Analysis 3.2.1 Proximate and Ultimate Analysis 3.2.2 Physical Tests . . . . . 3.3 Other Fuel Parameters 3.3.1 Particle Size and Shape 3.3.2 Charcoal and Char Properties 3.3.3 Biomass Ash Content and Effects 3.3.4 Biomass Moisture Content and Effects 3.3.5 Biomass Heating Value 3.4 Beneficiation of Biomass Fuels . . 3.4.1 Densifying Biomass Fuels 3.4.2 Drying Biomass Fuels 3.5 Biomass Fuel Emissions 4.0 Principles of Gasification 4.1 Introduction . . . . . . 4.2 Biomass Thermal Conversion Processes 4.2.1 Introduction . . . . . . 4.2.2 Biomass Pyrolysis . . . . 4.2.3 Combustion of Biomass . . . . 4.2.4 Chemistry of Biomass Gasification 4.2.5 Thermodynamics of Gasification .. . .

. 21 . 21 . 21 . 21 . 21 . 24 . 24 . 25

. 12 . 12 . 12 . 13 . 15 . 16 . 16 . 16 . 17 . 18 . 19

Contents

iii

4.3 Indirect and Direct Gasification Processes 4.3.1 Indirect (Pyrolitic) Gasification . 4.3.2 Direct Gasification . . . . . . ..

. 25 . 25 . 25 . . . . . 27 27 27 28 28

4.4 Principles of Operation of Direct Gasifiers 4.4.1 Introduction . '. . . . . . . . . . 4.4.2 Operation of the Updraft Gasifier 4.4.3 Operation of the Downdraft Gasifier 4.4.4 Factors Controlling Stability of Gasifier Operation 4.5 Charcoal Gasification 4.6 Summary .

. 28 . 29 . 30 . 30 . 30 . 31 . 31 . 32 . 32 . . . . . . 32 32 33 35 36 36

5.0 Gasifier Designs5.1 Introduction 5.2 Basic Gasifier Types 5.3 Charcoal Gasifiers . . 5.4 Charcoal versus Biomass Fuels 5.5 The Crossdraft Gasifier . . . . 5.6 The Updraft Gasifier . . . . . 5.7 The Imbert Downdraft Gasifier 5.7.1 Introduction . . . . . 5.7.2 Description of the Downdraft (Imbert) Gasifier 5.7.3 Superficial Velocity, Hearth Load, and Gasifier Sizing 5.7.4 Turndown Ratio . . . . . . . . . . 5.7.5 Disadvantages of the Imbert Design 5.8 The Stratified Downdraft Gasifier . . . . . 5.8.1 Introduction . . . . . . . . . . . . 5.8.2 Description of the Stratified Downdraft Gasifier 5.8.3 Unanswered Questions About the Stratified Downdraft Gasifier 5.8.4 Modeling the Stratified Downdraft Gasifier 5.9 Tar-Cracking Gasifiers 5.9.1 Introduction . . . . . 5.9.2 Combustion of Tars 5.9.3 Thermal Tar Cracking 5.9.4 Catalytic Tar Cracking 5.10 Summary . . . . . . . . . . .

. 38 . 38 . 38 . 40 . 42 . 42 . 42 . 43 . 45 . 46 . 46 . 48 . 48 . 48 . 48 . 49 . 49 . 49 . 49 . 49 . 49

6.0 Gasifier Fabrication and Manufacture6.1 Introduction . . . . . . . 6.2 Materials of Construction . . . . 6.3 Methods of Construction . . . . 6.4 Sizing and Laying out the Pipes 6.5 Instruments and Controls 6.5.1 Temperature 6.5.2 Pressure 6.5.3 Gas Mixture 6.5.4 Automatic Controls

iv

Handbook of Biomass Downdraft Gasifier Engine Systems

7.0 Gas Testing7.1 Introduction . . . . . . . . . . . . . . . . . . . 7.3 Description of Producer Gas and Its Contaminants 7.3.1 The Gas Analysis 7.3.2 Particulates 7.3.3 Tars . . . . . 7.4 Gas Sampling . . . . 7.4.1 Sample Ports 7.4.2 Isokinetic Sampling 7.5 Physical Gas-Composition Testing 7.5.1 Raw Gas . . . . . 7.5.2 Cleaned Gas 7.6 Chemical Gas Composition 7.6.1 Gas Samples for Chemical Analysis 7.6.2 Methods of Analysis . 7.6.3 Water Vapor Analysis . . . . . . . . 7.7 Analysis of Test Data . . . . . . . . . . . . . 7.7.1 Mass Balances and Energy Balances 7.7.2 Flow Rate Characterization . . . . 7.8 Particle-Size Measurement . . . . . . . . . 7.8.1 Typical Particle-Size Distributions 7.8.2 Sieve Analysis . . . . . . . 7.8.3 Microscopic Size Analysis 7.8.4 Aerodynamic Size Analysis 7.8.5 Graphic Analysis of Size Distribution 7.8.6 Physical Size Analysis 7.2 Gas-Quality Measurements and Requirements

. 51 . 51 . 51 . 51 . 51 . 51 . 55 . 55 . 55 . 56 . 57 . 57 . 61 . 61 . 61 . 62 . 65 . 66 . 66 . 67 . 67 . 67 . 67 . 67 . 67 . 69 . 70 . 71 . 71 . 72 . 74 . 74 . 74 . 74 . 74 . 74 . 74 . 75 . 75 . 75 . 80 . 83 . 84 . 84 . 86 . 88

8.0 Gas Cleaning and Conditioning8.1 Introduction . . . . . . . . . . 8.2 The Power Theory of Gas Cleanup 8.3 Gas Cleanup Goals . . . . . . . . 8.3.1 Gas Contaminant Characteristics 8.3.2 Typical Dirty Gas 8.3.3 Gas Cleanup Goals . . 8.3.4 Cleanup Design Target 8.4 Classification of Particles . . . 8.5 Particle Movement and Capture Mechanisms 8.6 Dry Collectors . . . . . . . . . . . 8.6.1 Gravity Settling Chambers 8.6.2 Cyclone Separators 8.6.3 Baghouse Filter 8.6.4 Electrostatic (Cottrell) Precipitators 8.7 Wet Scrubbers . . . . . . . . . . . . 8.7.1 Principles of Wet Scrubbers 8.7.2 Scrubber Equipment . 8.7.3 Auxiliary Equipment

Contents

v

8.8 Disposal of Captured Contaminants 8.8.1 Char-Ash . 8.8.2 Tar . . . . 8.8.3 Condensate

. 92 . 92 . 92 . 92 . 93 . 93 . . . . . 93 93 93 94 94

9.0 Gasifier Systems9.1 The Complete Gasifier System 9.2 Storing, Feeding, and Sealing Solids 9.2.1 Characteristics of Solids 9.2.2 Storage . . . . . . . 9.2.3 Feeding Solids . . . . . 9.2.4 Sealing Solid Flows . . 9.3 Fans, Blowers, Ejectors, and Compressors 9.3.1 Importance of Gas-Moving System Design 9.3.2 Fans 9.3.3 Blowers 9.3.4 Ejectors . 9.3.5 Turbochargers and Superchargers 9.4 Flares and Product-Gas Burners 9.4.1 Flares . 9.4.2 Burners . . . . . .

. 95 . 95 . 95 . 96 . 96 . 97 . 97 . 97 . 98 . 99 . 99 . 99 . 99 100 103 103 103 103 103 104 104 105 105 105 105 105 105 106 106 106 106 106 106

1 0.0 Instrumentation and Control10.1 The Need for Instruroentation and Control 10.2 Gasifier Instruroents . . . . . . . 10.2.1 Pressure Measurement . . 10.2.2 Gas Flow Measurement 10.2.3 Solid Flow Measurement 10.2.4 Temperature Measurements 10.3 Controls . . . . . . . . . . . 10.3.1 Fuel-Level Controls 10.3.2 Pressure Controls . . 10.3.3 Temperature Controls 10.4 Computer Data Logging and Control

1 1 .0 Engine Adaptation and Operation11.1 Introduction . . . . . . . . . . . 11.2 Producer Gas for Transportation 11.3 Producer Gas for Electric Power and Irrigation 11.4 Gasifier Types Suitable for Shaft-Power Generation 11.5 Sizing the Gas Producer to the Engine . . . . . . . 11.6 Engine Selection . . . . . . . . . . . . . . . . . . . 11.6.1 Large-Vehicle Engines - Truck Engines up to 50 kW 11.6.2 Small Engines 11.6.3 Natural-Gas Engines 11.6.4 Diesel Engines 11.7 Cogeneration . . . . . . . . .

vi


11.8 Spark-Ignition Engine Conversion 11.8.1 Engine System 11.8.2 Gas Mixers 11.8.3 Power Time Lag 11.8.4 Engine Startup 11.8.5 Ignition Timing 11.8.6 Spark Plugs . . 11.9 Two-Cycle Engine Conversion 11.10 Diesel Engine Conversion . . 11.10.1 Diesel Operation with Producer Gas 11.10.2 Starting Diesel Engines . . . . . . . 11.10.3 Throttling at Partial Load . . . . . . 11.11 Increasing Power from Producer-Gas-Fueled Engines 11.11.1 Mechanisms of Power Loss 11.11.2 Engine Breathing . . . . . ' 11.11.3 Efficiency and Power Loss 11.11.4 Blowers and Superchargers 1 1 . 11.5 Other Methods for Increasing Producer Gas Power 11.12 Engine Life and Engine Wear 11.12.1 Engine Life Expectancy . . . . . . . . . . . . . . 11.12.2 Sticking Intake Valves 11.12.3 Oil Thickening and Contamination 11.12.4 Tar/Oil Accumulations 1 1 . 12.5 Engine Corrosion 11.12.6 Engine Warranty 1 1 . 13 Exhaust Emissions . . . .

. . . . . . . . . . . . . . . . : . . . . . . . . . . .

107 107 107 108 109 110 110 111 111 112 113 113 113 113 114 114 115

. 110

115 115 116 116 1 16 116 117 117 117 117 118

. 117

11.14 Other Devices for Producer-Gas Power Generation 11.14.1 Gas Turbines . . . . . . . . . . 11.14.2 Fuel Cells . . . . . . . . . . . 11.14.3 External-Combustion Devices

1 2.0 Safety and Environmental Considerations12.1 Introduction . . . . . . . 12.2 Toxic Hazards . . . . . . 12.2.1 Carbon Monoxide 12.2.2 Creosote . . . . 1 2 . 3 Fire Hazards . . . . . . 12.4 Environmental Hazards

. 119 . 119 . 119 . 119 . 121 . 122 . 123 . 124 . 124 . . . . . . . 124 124 124 124 124 124 124

13.0 Decision Making . . . . .13.1 Introduction . . . . . 13.2 Logistics Assessment 13.2.1 Gasifier Application 13.2.2 Equipment Selection Factors 13.2.3 Feedstock Supply 13.2.4 Regulations . . . . . . . . . . . . . . . . . 13.2.5 Labor Needs 13.2.6 Final Logistics Considerations

Contents

vii

13.3 Economics . . . . . . . . . . . . 13.3.1 Costs . . . . . . . . . . 13.3.2 Calculating Energy Costs 13.3.3 Equipment Cost 13.3.4 Conversion Efficiency and Fuel Consumption 13.3.5 The Cost of Operating Labor 13.3.6 Maintenance Costs . . . . 13.4 Cost Benefits . . . . . . . . . . . . 13.4.1 Value of Power Produced 13.4.2 Cogeneration Possibilities 13.5 Financing . . . . . . . . . . . . . 13.5.1 Government Subsidies in the Form of Tax Incentives 13.5.2 Financial Institutions 13.6 Other Considerations

125 125 125 126 127 127 127 128 128 129 129 129 129 129 131

References Appendix .

139

viii


Chapter 1 Introduction and Guide to the Literature and Research

1 .1 Role of Gasification in Biomass Conversion

This handbook explains how biomass can be converted to a gas in a downdraft gasifier and gives details for designing, testing, operating, and manufacturing gasifiers and gasifier systems, primarily for shaft power generation up to 200 kW. It is intended to help convert gasification from a practical art into a field of en gineered design. Although the handbook focuses on downdraft gasification as the only method suitable for small-scale power systems, it also gives extensive detail on biomass fuels, gas testing and cleanup in strumentation, and safety considerations that will be of use to all those who work with gasifiers at whatever scale. The combustion of biomass in wood stoves and in dustrial boilers has increased dramatically in some areas, and forest, agricultural, and paper wastes are being used extensively for fuels by some industries. However, more extensive biomass use still waits for the application of improved conversion methods, such as gasification, that match biomass energy to processes currently requiring liquid and gaseous fuels. Examples of s uch processes include glass, lime, and brick manufacture; power generation; and transportation. Biomass, like coal, is a solid fuel and thus is inherent ly less convenient to use than the gaseous or liquid fuels to which we have become accustomed. An over view of various processes now in use or under evalua tion for converting biomass to more conventional energy forms such as gas or liquid fuels is shown in Fig. 1-1 (Reed 1978). The figure shows how sunlight is converted to biomass through either traditional ac tivities (e.g., agriculture and silviculture) or new in novative techniques (e.g., as energy plantations, coppicing, and algaeculture) now being developed. Biomass resources fall into two categories: wet or wet table biomass (molasses, starches, and manures) and dry biomass (woody and agricultural materials and residues). Biological processes require wet biomass and operate at or near room temperature. These proces ses, shown on the lower left side of Fig. '1-1, include fermentation to produce alcohols and digestion to produce methane. Thermal processes function best using biomass feedstocks with less than 50% moisture content and are shown on the right side of Fig. 1-1. The simplest thermal process is combustion, which yields only heat. Pyrolysis uses heat to break down biomass and yields charcoal, wood-oils, tars, and gases.Gasification processes convert biomass into combus tible gases that ideally contain all the energy original ly present in the biomass. In practice, gasification can convert 60% to 90% of the energy in the biomass into energy in the gas. Gasification processes can be either direct (using air or oxygen to generate heat through ex othermic reactions) or indirect (transferring heat to the reactor from the outside). The gas can be burned to produce industrial or residential heat, to run engines for mechanical or electrical power, orto make synthetic fuels.

In one sense, biomass gasification is already a well proven technology. Approximately one million downdraft gasifiers were used to operate cars, trucks, boats, trains, and electric generators in Europe during World War II (Egloff 1943), and the history of this ex perience is outlined in Chapter 2. However, the war's end saw this emergency measure abandoned, as inexpensive gasoline became available (Reed 1985b). Development of biomass gasification was disrupted in 1946 as the war ended and inexpensive (15/gal) gasoline became available. The magnitude of damage inflicted on gasifier technology by this disruption Can be seen by the fact that it is difficult for even the "ad vanced" technology of the 1980s to achieve on tests what was routine operation in the 1940s. The design, research, and manufacturing teams of that decade have all disbanded. We have from the past only that small fraction of knowledge that has been published, whereas the large bulk of firsthand experience in operation design has been lost and forgotten. Gasification was rediscovered in an era of fuel shortages and higher oil prices, and there are gasifier engine projects under way in more than 20 countries for producing process heat and electrical and mechani cal power (Kjellstrom 1983, 1985). In its rebirth, however, the existing technology has uncovered major problems in connection with effluent and gas cleanup and the fuel supply, which were less important during the emergency of World War II. Today, these problems must be solved if biomass gasification is to reemerge a a fuel source. Apparently, it is going to take a few years for the technology of the 1980s to be effectively applied to the accomplishments of the 1940s. Space-age advan ces in materials and control systems are available for

Introduction and Guide to the Literature and Research

use in today's process designs, so a continuous development effort and lively open exchange should enable us to incorporate latter-day chemical and chemical engineering techniques to build clean, con venient, and reliable systems. A recent workshop on low-energy gasification tabulates research and development needs (Easterling 1985). The accelerated use of gasification technologies ul timately depends upon their ability to compete with fossil fuels, which in turn depends on unknown factors about resources, economics, and political conditions. At present (1988), gasification and other alternative energy processes are being developed slowly in the United States because of relatively plentiful supplies of low-cost gaseous and liquid fossil fuels. However, political changes could rapidly and dramatically alter this situation, as witnessed during the OPEC oil crises

of the seventies. The U.S. Office of Technology Assess ment (OTA) recently has issued a report calling for a national capability for emergency implementation of gasifiers (OTA 1984).

1 .2 Biomass Energy PotentialBiomass is a renewable fuel that supplies 2% to 3% of U.S. energy needs and an even larger percentage in some other countries (OTA 1980; DOE 1982). OTA projects that biomass could supply from 7% to 20% (6 17 quads*) annually (OTA 1980) from sources such as those shown in Table 1-1 (Reed 1981), if it can be made available in a convenient form and if conversion equip ment is accessible. The potential of biomass for world use is equally great (Bioenergy 1985).*1 quad=

1015

Btu

Carbon dioxide1-1 IL

Product farming (existing) Industry Agriculture Silviculture Farm and forest products Municipal wastesI

'\

/s

unligh

Water

J

Energy farming (potential) Aquaculture Silviculture AgricultureBiomass for energy

Land I

L

I Maceration r Drying and densification II I

Residues t-

J

I

Extraction Digestion Chemicals Methane (rubber) (cattle fed) (resins)

I

Bioconversion processes (wet)

I Needs I I Chemicals

Thermal conversion processes (dry) Fermentation I I and distillation Liquefaction Combustion Pyrolysis 1-------1 I Gasification Ethanol f{ Air I Oil gas Heat Oil gas (sugars) charcoal systems tu L---I I LO';;a I fJxygentI Med.- B tu gas methanol ammonia' I___

Gaseous fuels

Liquid fuels

Solid fuels

Electricity

Heat

Fig. 1-1. Biomass energy paths (Source: Reed 1978)

2


Table 11 . Summary of the Annual Energy Potential of Existing Sources of Biomass in the United States

Crop residues

Resource

106

Dry Tons/Year

Quads/Year

Animal manures Unused mill residuesa Logging residues Munic ipa l solid wastes Standing forests Totals

278.0 26.5 24.1 83.2 130.0 384.0 925.8

4.15 0.33 0.41 1.41 1.63 6.51 14.44

1000 writers and workers in the field. Unfortunately, massive bibliographies of undifferentiated material can confuse the reader or give an impression of a level of understanding that does not exist for gasification. We hope this manual will help the reader to put this material into perspective.

1 .3.2 BooksThere was a great deal of research and commercializa tion directed toward coal and biomass gasification be tween 1850 and 1950. However, cheap and plentiful gas and oil prevented the commercial development of the technology except in times of emergency. The reader is referred especially to a number of excellent historical books. Modern Gas Producers (Rambush 1923) gives an account of experiences with updraft and coal gasifiers. Generator Gas (Gengas 1950) and its se quel, Wood Gas Generator for Vehicles (Nygards 1979), give the reader a complete coverage of all aspects of downdraft gasifiers during World War II. Gas Producers and Blast Furnaces (Gumz 1950) looks at the ther modynamics and kinetics of coal and wood gasifica tion. The article by Schlapfer and Tobler, "Theoretical and Practical Studies of Operation of Motorcars on Wood Gas," (Schlapfer 1937) is the best practical and scientific discussion of small gasifiers to appear during that period. A more general survey of biomass thermal conversion was published during 1979-80 in the SERI three volume Survey of Biomass Gasification (Reed 1981). This work subsequently was published commercially as Principles of Biomass Gasification (Reed 1981). The work Producer Gas: Another Fuel for Motor Transport (NAS 1983) contains an excellent historical perspec tive as well as a projection of coming developments. A monumental work, Small-Scale Gas Producer Engine Systems, is available in the United States and Germany (Kaupp 1984a). In addition to other considerations, this work contains an in-depth treatment of the use of forest and agricultural residues. Finally, several private groups have published or republished gasifier plans or gasifier books and pamphlets (TIPI 1986; Skov 1974; Mother 1982; Nunnikhoven 1984; Nygards 1979).

aDoes not include unused bark from wood pulp mills. Source: Reed 1981. p. 39Biomass is a renewable energy form with many posi tive features. The biomass feedstock is often a low-cost byproduct of agriculture or silviculture; it is low in ash and sulfur content, and it does not increase the level of carbon dioxide in the atmosphere and the subsequent greenhouse effect (provided that consumption does not exceed annual production). Care must be taken to en sure that biomass use as fuel is on a renewable basis (Lowdermilk 1975; Reed 1978). Today, many countries (such as China, Korea, Brazil, and South Africa) have active reforestation programs that are helping to in crease the total world forest area. With continued diligence, the prospects for making biomass truly renewable will steadily improve.

1.3 Guide to Gasification Literature

1 .3.1 BibliographiesThe number of books, articles, and reports on biomass gasification easily exceeds 10,000 (Reed 1985b), with many important studies conducted before 1950. One can easily become discouraged when trying to find the earlier works. Fortunately, much of this early work has been collected; some of it has been summarized, and some of it has been reprinted. We offer here an over view of this body of knowledge in order to help the reader locate required material. In general, the more recent works are still available. Two major collections of the older papers have been made in the past decade. The U.S. National Academy of Sciences published a bibliography of its extensive collection of early papers in Producer Gas: Another Fuel for Motor Transport (NAS 1983). The University of California at Davis acquired an extensive collection of papers while preparing State of the Art for Small Gas Producer Engine Systems (Kaupp 1984a). Most of these papers are also in the possession of A. Kaupp at GATE in Germany and also are on file at SERI. A very recent publication from India, State of Art Report on Biomass Gasification, (Parikh 1985) contains more than 1200 abstracts of articles on gasification as well as an assess ment of its viability and an excellent list of more than

1 .3.3 Gasification ProceedingsCurrent gasification work generally is reported at con ferences and then appears in the published proceed ings. The U.S. Department of Energy (DOE) (PNL 1982; Easterling 1985) the U.S. Department of Agriculture (USDA), the Forest Products Research Society (FPRS 1983], the U.S. Environmental Protection Agency (EPA], and the Institute of Gas Technology (IGT) all have had continuing interest in various forms of gasification and have sponsored conferences dealing with this field. These publications contain many

I ntroduction and Guide to the Literature and Research

3

articles of interest, and the proceedings often span many years of research. The Electric Power Research Institute (EPRI) has commissioned two studies on the use of producer gas (Miller 1983; Schroeder 1985). Govermnent interest in gasification has tended to focus on large-scale systems. Biomass gasification is perceived by the foreign aid agencies of the developed countries (such as the U.S. Agency for International Development [U.S. AIDlJ as a major potential energy source for many parts of the developing world. The Beijer Institute of Sweden has organized two international conferences for these donor agencies and published three volumes of recent studies of gasification relevant to the problems of developing countries (Kjellstrom 1983, 1985). South Africa is uniquely situated relative to producer gas research because it is highly developed technical ly and produces much of its fuel by gasification. However, it also has a native population of 20 million whose needs match those of less developed couritries. A major world conference in timber utilization in May 1985 included week-long sessions on both wood gasification and charcoal manufacture (NTRI 1985). The European Economic Community (EEC) has shown a great deal of interest in biomass energy in all forms and has been very active in gasification during the last five years (CEC 1980, 1982; Bridgwater 1984; Bioener gy 1985). The EEC has focused on the high-tech aspects of gasification (such as oxygen gasification), but has also funded work in small-scale gasifiers as part of its perceived responsibility toward "associated" develop ing countries (Beenackers and van Swaaij 1982; Carre 1985; Bridgwater 1984; NTRI 1985; Manurung and Beenackers 1985).

research group in producer gas (IGT 1984). In addition, excellent gasification work is proceeding in Canada, Europe, Brazil, the Philippines, New Zealand, and other parts of the world, primarily at the university level.

1 .3.6 Producer Gas R&D FundingU.S. AID has had a strong interest in producer gas tech nology because it offers a means for reducing the de pendency of developing nations on imported fuels and has supported a number of projects around the world. The Producer Gas Roundtable of Stockholm, Sweden, is an oversight organization supported by various in ternational development agencies to promote informa tion exchange on gasification, to and between developing countries. It has sponsored two major in ternational conferences (Kjellstrom 1983, 1985). A moderate level of funding ($2 million to $5 mil lion/yr) has been maintained since 1975 by DOE for "advanced concept" gasification and pyrolysis pro cesses. Most of the work is aimed at large industrial processes and is supported in government laboratories, industrial firms, and universities. Progress in these. programs is reported at the meetings of DOE's Ther mochemical Conversion Contractors (PNL 1986), as well as at other meetings. DOE recently sponsored a meeting to examine the potential and problems of low energy gasification (Easterling 1985) but is currently focusing on direct liquefaction of wood. The status of many of the government research and development projeCts and commercial gasifiers projects was sum marized in SurveyofBiomass Gasification (Reed 1981). EPRI (Schroeder 1985) has evaluated the potential of gasifiers for making electricity. The Forest Service of the USDA holds annual meetings at which gasifiers are discussed (FPRS 1983). Reports on government programs are maintained by the Office of Scientific and Technical Information (OSTIl where they can be obtained in either microfiche or printed copies. They are sometimes difficult to obtain after the original supply of reports is exhausted. Copies of these reports are also available in GPO depository libraries. There are at least two such libraries-one public and one university-in each state.

1 .3.4 Commercial InformationAnother source of gasifier information is provided by companies developing commercial gasifier systems. These groups write advertising brochures as often as they write scientific articl s, and it is sometimes difficult to separate actual from projected performance. Their publications should be read critically but usually contain important (if optimistic) information.

1 .3.5 Producer Gas ResearchMuch research into air gasification is being conducted at various universities around the world. However, it is difficult to trace this work if it is occurring either un funded or on a small scale. The work of Goss and his students at the University of California at Davis de serves special mention because it has spanned a decade and includes both experimental and theoretical studies (Goss 1979). Twente University in the Netherlands has had a large program in gasification for many years (Groeneveld 1980a,b; Aarsen 1985; Buekens 1985). The University of Florida at Gainesville has a very active

1 .3.7 Federal Emergency Management Agency (FEMA) Gasifier WorkThe downdraft gasifier reached its highest develop ment during the emergency of World War II. FEMA has taken interest in small-scale gasifiers because they could function during a period of breakdown in our oil supply due to atomic attack or other disruption of conventional fuels.

4


With this i n mind, FEMA contracted with H. LaFontaine of the Biomass Energy Foundation to build a prototype gasifier that could be made with readily available parts and to write a "craftsman

manual" description of gasifier construction and operation (LaFontaine 1987). The gasifier has passed the test, and the manual is now in the process of being published by FEMA.

Introduction and Guide to the Literature and Research

5

Chapter 2 History, Current Developments, and Future Directions

2.1 Historical Developmenttrucks, cars, and buses in Europe and probably more than a million worldwide (Egloff1943). However, these impressive numbers included only six wood-fuele.d . vehicles in the United States and two in Canada, where low-cost gasoline continued to be available throughout the war. Many articles were written on gasification during that time (see Chapter 1). Some photographs of gasifiers fitted to vehicles of that era are shown in Fig. 2-1. Most gasifiers were simply "belted on" and

2.1.1 Early Development of GasificationGasification was discovered independently in both France and England in 1798, and by 1850 the technol ogy had been developed to the point that it was pos sible to light much of London with manufactured gas or "town gas" from coal (Singer 1958; Kaupp 1984a). Manufactured gas soon crossed the Atlantic to the United States and, by 1920, most American towns and cities supplied gas to the residents for cooking and lighting through the local "gasworks." In 1930, the first natural gas pipeline was built to transport natural gas to Denver from the oil fields of Texas. As pipelines crisscrossed the country, very low cost natural gas displaced manufactured gas, and the once-widespread industry soon was forgotten. "Town gas" continued to be used in England until the 1970s, but the plants were dismantled following the discovery of North Sea oil. Today, a few plants are still operating in the third world.

2.1 .2 Vehicle GasifiersStarting about the time of World War I, small gasifiers were developed around charcoal and biomass feedstocks to operate vehicles, boats, trains, and small electric generators (Rambush 1923). Between the two world wars, development was pursued mostly by amateur enthusiasts because.gasoline was relatively in expensive and simpler to use than biomass. In 1939 the German blockade halted all oil transport to Europe. Military use of gasoline received top priority, and the civilian populations had to fend for themselves for transport fuels. Approximately one million gasifiers were used to operate vehicles worldwide during the war years. The subsequent development of wood producer gas units is a testament to human ingenuity in the face of adversity. Extended accounts make fas cinating reading and inform the reader of both the promise and difficulties of using producer gas. (Egloff 1941, 1943; Gengas 1950; NAS 1983; Kaupp 1984a). At the beginning of World War II, there was a great deal of interest in all forms of alternative fuels (Egloff 1941, 1943). By 1943, 90% of the vehicles in Sweden were powered by gasifiers. By the end of the war, there were more than 700,000 wood-gas generators powering

Fig. 2-1. Vehicle gasifiers before 1950 (Source: NAS 1983)

6


regarded as only temporary modifications for wartime conditions. However, a few car makers went so far as to modify the body work for gasifier installation. Soon after the war, low-cost gasoline became available again, and most users went back to burning gasoline because of its convenience.

2.3 Future Development DirectionsPredicting the needs and direction of development in our modern world is very dangerous, because we don't know how future conditions will change and what our response will be. Since the first OPEC embargo in 1973, we have oscillated between a concern with energy sup plies and business as normal. Therefore, we can't predict which direction we are likely to go, but we can at least list the possible options and factors that affect the choice. In normal times, development is driven by economic considerations, and some of the economic factors in fluencing use of gasification are listed in Chapter 13. In times of emergency, om priorities change drastically and quite different developments occur. Small gasifiers were developed very rapidly during the emergency of World War II and just as rapidly disap peared when liquid fuels were available. Transporation is a very high priority, and the U.S. Department of Defense currently has a program to disseminate infor mation on small gasifiers in case of national emergency. However, for economic reasons, no work on gasifiers for vehicles is in progress in the United States. During the late 1970s, we imported more than 40% of our oil. We reserved much of our liquid fuel for transport, and there was no government call to develop gasifiers in the United States. (However, Sweden-Volvo manufactured and stored 10,000 units for emergency use.) In the private sector ofthe United States during the last 10 years, there has been a corresponding development of biomass gasifiers for heat applications at the scale found in lumber and paper mills. There has been inter est in power generation at a small scale in the United States stimulated by attractive power buy back rates in some states under the Public Utilities Regulatory Policy Act (PURPA) discussed in Chapter 13. A very active area of development for small gasifiers is to generate power in developing countries, which have biomass resources and cannot easily afford liquid fuels. They do not have an electrical distribution grid so power systems of 10 to 1000 kW are very attractive. Thus, the scale of operation has an important influence on what is developed in this case. Finally, new developments in gasifiers may extend their use to other new areas. One of our authors (Das) has developed a small gasifier suitable for firing a foundry. The other author (Reed) is developing small batch-type gasifiers for cooking and lighting applica tions in third world countries.

2.2 Current Development ActivitiesAfter the OPEC oil embargo of 1973, there was renewed interest in all forms of alternative energy, including gas produced from coal and biomass. Most of the early work supported by the United States and foreign energy establishments focused on large-scale coal-fed gasifiers that were intended to produce synthetic natmal gas as a fuel. There was little interest in biomass or biomass gasification (PNL 1986). except for groups concerned with uses in less developed countries (NAS 1983; Kjellstrom 1981, 1983, 1985) and private individuals (Skov 1974; Mother 1982; TIPI 1986). Recently, there has been increased interest in biomass as a renewable energy source. In the last few years, a number of individuals and groups have built versions of small downdraft gasifiers and have operated them as demonstration units. A few of the gasifier-powered vehicles from this effort are shown in Fig. 2-2, and today one can obtain shop plans for constructing gasifiers (Nunnikhoven 1984; Mother 1982; Skov 1974). Unfortunately, no body of information is avail able to help either the latter-day hobbyists or their counterparts involved in full-time research to evaluate critical factors such as gasifier operation, gas quality, gas-cleanup systems, engine operation, and engine wear. Interest in small-scale gasifiers is strong among or ganizations that deal with less developed countries such as the World Bank, the U.S. Agency for Interna tional Development, and the equivalent organizations in European countries. The Producer Gas Roundtable (of the Beijer Institute in Stockhohn) has published a number of books on gasification and drawn together technical expertise from around the world. In addition, this group has hosted several conferences on producer gas for less developed countries (Kjellstrom 1981, 1983,1985). Producer gas from charcoal has been developed com mercially in the Philippines (Kjellstrom 1983), where more than 1000 units have operated. Producer gas is generated for industrial heat by more than 30 large units operating in Brazil (Makray 1984).

History, Current Developments, and Future Directions

7

Fig. 2.2. Vehicle gasifiers after OPEC (Source: NAS 1983)

8


Chapter 3 Gasifier Fuels

3.1 IntroductionBiomass fuels occur in a multitude of physical forms. The often-heard manufacturer's claim that a particular gasifier can gasify "any biomass fuel" is a naive state ment, and each form can be expected to have unique problems until proven otherwise. This physical dis parity accounts in part for the large number of gasifier designs available today. The gasifiers used widely during World War II used specially prepared 1x2x2 cm3 hardwood blocks. However, such blocks could repre sent only a tiny fraction of the biomass materials avail able for gasification. Some gasifiers currently are undergoing design evolutions that will enable them to use a wider range of fuels; nevertheless, fuel properties are very important in determining satisfactory operat ing conditions. Therefore, these multifeedstock gasifiers will be able to use only a limited range of biomass with controlled specifications, and anyone in stalling such a gasifier should have tests run on the fuel to b e used before deciding upon a purchase. The ability to specify fuel parameters is very important, and we discuss them in this chapter. Fortunately, a wide variety of tests are available for biomass and charcoal gasifiers that can be useful to those interested in gasification. Green wood can contain up to 50% water by weight, so its properties vary widely with moisture content. The chemical composition of biomass (expressed on a dry, ash-free basis) is more constant than that of the various coals (bituminous, anthracite, lignite) as shown in Fig. 3-1. Furthermore, more than 80% of the biomass is volatile. Coal is typically only 20% volatile; the remaining 80% is unreactive coke, which is more dif ficult to gasify than charcoal. Biomass generally has very low sulfur and ash content compared to coal. However, unlike coal, biomass comes in a wide variety of physical forms, making it necessary to tailor the shapes of the gasifier, fuel-drying equipment, feed sys tems, and ash-removal equipment to each form. There for e, the resulting gasifier design must be very fuel-specific. in detail in the publications of the American Society for Testing Materials (ASTM), shown in Table 3-1. The equipment necessary for p erforming elemental analysis is shown in Table 3-2. The proximate analysis

o

1 0,000 Ii; 8000 !IS 6000 -;; 4000, .; 2000Q. U

Calorific Value of the dry fue/s

=

Oxygen Hydrogen C CarbonH= =

100 80""

O

-----

__

__ __

__

-J

1;: 3':'iii

OJ

.

60 40 20

o

'"

J'l

. CO + 0.7 H2

CH, . 400.6 + 1.05 02 + (3.95 N2 )

0.7 H20 + (3.95 N2)

(4-1)

where CH1 .400 .6 is an average formula for typical biomass. (Actual composition for specific biomass is shown in Tables 3-3. 3-4. 36. and 3-7). The nitrogen is shown in parentheses because it is an inert portion of air and does not take part in the reaction. For oxygen combustion of biomass it would be omitted. This combustion produces 20.9 kJ/g (8990 Btu/lb) when the temperature of the combustion products is low enough for all the liquid to be water. and this is the value that would be measured in a bomb calorimeter and reported as the high heat of combustion or HHV as shown in Tables 3-6 and 4-1. In most practical combus tion devices. the water escapes to the atmosphere as a gas. and the heat of vaporization of the water is not recovered. In this case. the low heating value. LHV. 20.4 kJ/g (8770 Btu/lb). would be the maximum heat that could be generated. The difference between LHV and HHV is small for dry wood but increases rapidly with moisture content of the wood. (In the United States the HHV is normally used for rating the

(4-2)

Unfortunately. there is more energy contained in the CO and H2 than is contained in the biomass. so that this reaction would require the transfer of energy from some external source, which would greatly complicate the process. In practice. some excess oxygen must then be added for gasification (carrying the reaction to point in Fig. 4-1(bll. producing some CO2 and H20 according toCH1 .400. 6 + 0.4 02->

0.7 CO + 0.3 CO2

+

0.6 H2 + 0.1 H20

(4-3)

Typically a few percent of methane are formed as well. Typical properties of producer gas from biomass are shown in Table 4-2.

Table 4-2. Typical Properties of Producer Gas from Biomassa

Compound Symbol Gas (vol.%) Dry Gas (vo l .%)

Table 4-1. Thermal Properties of Typical BiomassTypical dry biomass formula: (moisture- and ash-free [MAFJ basis) CH 1 .400.6 C H 4.3 52.2 Composition (weight %) 46.7 33.3 Composition (mole %) High Heating Valuea Low Heating Value

41 .7 20.0

0

20.9 kJ/g (8990 Btu/lb) 20.4 kJ/g (8770 Btu/lb)

Carbon monoxide Carbon dioxide Hydrogen Water (v) Methane N itrog en

aThe high heating value (HHV) is the value that is usually measured in the laboratory and would be obtained during combustion if liquid water was allowed to condense out as a liquid. The low heating value (LHV) is obtained when water is produced as a vapor. The high heating value of typical biomass fuels will be decreased in proportion to the water and ash content, according to the relation: LHV(Net) HHV(MAF)/(1 + M + A) where M is the fraction of moisture (wet basis), A is the fraction of ash, and MAF designates the moisture- and ash-free basis. The air/biomass ratio required for total combustion is 6.27 kg/kg (Ib/lb). The LHV can be related to the HHV and an analysis of the combus tion products as: HHV LHV + Fm hw where Fm is the weight fraction of moisture produced in the combus tion gases, and hw is the heat of vaporization of water, 2283 Jig (980 Btuilb). Source: Modified from data in Reed 1981.= =

22.1 CO 21.0 1 0.2 CO2 9.7 14.5 H2 1 5.2 4.8 H2O 1 .7 1 .6 CH4 N2 50.8 48.4 Gas High Heating Value: 5506 kJ/Nm3 (1 35.4 Btu/scf) Generator gas (wet bas is )b 5800 kJ/Nm3 (142.5 Btu/scf) Generator gas (dry basis) bAir Ratio Required for Gasification: 2.38 kg wood/kg air (Ib/lb)

Air Ratio Required for 1 .1 5 kg wood/kg air (Ib/lb) Gas Combustion:

8These values are based on ash- and moisture-free bir:-mass with the composition given in Table 4-1. The wet-gas composition is the most important property of the gas for mass and energy balances, but the dry-gas composition is usually reported because of the difficulty in measuring moisture. The heating value of the gas is usually calculated from the gas composition, using a value of 1 3,400 kJ/Nm3 (330 Btu/sc for H2 and CO. and 41 .900 kJ/Nm3 (1030 Btu/sci) for methane. bThese are typical values for downdraft air gasifiers, but they can vary between 4880 and 7320 kJ/Nm3 (120-180 Btu/scf). depending on vari ables such as gasifier heat loss, biomass moisture content, and char removal at the grate. Source: Modified from data in Reed 1981.

24


The ratio CO/COz (or Hz/HzO) is a measure of the producer gas quality. Approximately 30% of the biomass is burned to provide the energy for gasification of the rest. The exact amount of excess oxygen required depends on the efficiency of the process. It can be im proved in practice with insulation, by drying, or by preheating the reactants. A fascinating question in gasification is how the reacting products "know" how much oxygen to use (see below).

of about 0.25 all of the char is converted to gas, and the fraction of energy in the wood converted to gas reaches a maximum. With less oxygen, some ofthe char is not converted; with more oxygen, some of the gas is burned and the temperature rises very rapidly as shown in Fig. 4-4(a), Thus, it is desirable to operate as close to an equivalence ratio of 0.25 as possible. How is it possible to operate exactly at this ratio ofO.25? In a fixed bed gasifier, operation at lower values of would cause charcoal to be produced (as shown for low in Fig. 4-4(c)), and it would build up in the reactor unless it is augered or shaken out. Operation at values of above 0.25 consumes charcoal and the temperature goes up rapidly. Hence, maintaining the bed at a con stant level automatically ensures the correct oxygen input.

4.2.5 Thermodynamics of GasificationThermodynamics is the bookkeeping of energy. Al though thermodynamics cannot always predict what will happen for a particular process, it can rule out many things that cannot happen. It was mentioned above that Eq. (4-2) is thermodynamically impossible in the absence of added heat and that Eq. (4-3) actual ly governs the reaction. How is this determined? At the high temperature where gasification takes place (typically 70oo-1000C), there are only a few stable combinatio::ts of the principal elements of biomass carbon, hydrogen, and oxygen. These are C, CO, COz, CH4, Hz, and HzO. The relative concentration of these species that will be reached at equilibrium can be . predicted from the pressure, the amount of each ele ment, and the equilibrium constant determined from the thermodynamic properties and temperature, sub ject to an energy balance. It is then possible to deter mine the species that would form at equilibrium as a function of the amount of oxygen added to the system. The results of calculations of this type are shown in Figs. 4-4 and 3-5. The adiabatic reaction temperature of biomass with air or oxygen, determined in this manner, is shown in Fig. 4-4(a). This is the temperature that would be reached if biomass came to equilibrium with the specified amount of air or oxygen. (There is no guaran tee that equilibrium will be reached in any given gasifier, but downdraft gasifiers approach equilibrium quite closely - see below,) The oxygen used in a process determines the products and temperature of the reaction. The oxygen consumed is typically plotted as the equivalence ratio, - the oxygen used relative to that required for complete com bustion. (Complete oxidation of biomass with oxygen requires a weight ratio of 1.476 [mass of oxygen/mass ofbiomassl; with air, a ratio of 6.36.) A very low or zero oxygen use is indicative of pyrolysis, shown at the left ofthe figure; a of about 0.25 is typical ofthe gasifica tion region at the middle; and combustion is indicated by a :2 1 at the right. The composition of the gas produced is shown in Fig. 4-4(b). The amount of energy remaining in the char and converted from solid to gas is shown in Fig. 4-4(c). The low heating value ofthe gas is shown in Fig. 4-4(d). From these figures it is seen that at an equivalence ratio

4.3 Indirect and Direct Gasification Processes

4.3.1 Indirect (Pyrolitic) GasificationIt is now recognized that wood-oil vapor is unstable at temperatures above 600C and cracks rapidly at 700 to SOOC to form hydrocarbon gases (such as methane, ethane, and ethylene), Hz, CO, and COz' In addition, one obtains a 1% to 5% yield of a tar composed of polynuclear aromatics and phenols similar to those found in coal tar (Antal 1979; Diebold 19S4; Diebold 19S5), Pyrolytic gasification is accomplished when a portion of the fuel or char is burned in an external vessel with air, and the resulting heat is used to supply the energy necessary to pyrolyze the biomass. The principal ad vantage of this process is that a medium-energy gas is produced without using oxygen. The higher energy content may be required for long-distance pipeline delivery. The disadvantage is that a significant fraction of tar may be produced, and indirect heat or mass trans fer is required, which complicates the apparatus and the process. Pyrolytic gasification will not be discussed further because it is only practical in large installations and is not as well-developed as direct gasification with oxygen or air.

4.3.2 Direct GasificationPyrolysis and gasification processes are endothermic, so heat must be supplied in order for the processes to occur. In fact, the heat required to accomplish pyrolysis and raise the products to 600C is about 1.6-2.2 kJ/g (700-S00 Btu/lb), representing 6% to 10% of the heat of combustion of the dry biomass (Reed 19S4), This heat is supplied directly by partially combusting the volatile tars in downdraft gasifiers; in updraft gasifiers, it comes from the sensible heat of the gases resulting from charcoal gasification. This combustion then dilutes the product gas with COz and HzO, the products

Principles of Gasification

25

of combustion with oxygen. If the combustion is ac complished with air. the gas is also diluted with about 50% nitrogen from the air. The principal advantages of direct gasification are that the one-stage process is very simple; the direct heat transfer from the gases to the biomass is very efficient.

and the process is largely self-regulating. If air is used. the resulting gas is diluted with atmospheric nitrogen to a producer gas value of 5800-7700 kJ/Nm3 (150 200 Btu/scf). When oxygen is used for gasification. a medium-energy gas containing 1 1 . 5 0 0 kJ/Nm3 (300 Btu/scf) is obtained (Reed 1982). Medium-energy gas can be distributed economically for short distances

3000

P=1atm

3000

20

1816'CC>

g

14 12

0-

Energy in Gas"", ,

P l aIm

300!!!u: u

:;, E' .. "w

-;: 10 "" 8 64

2 0 0

0,

\ Air\ \0.2

Energy in Char

Air

,0, ....... ....... ....... .......

0;::>

III

>

'"

200

Oxygen Air

P = l aIm

c: ."

C> '" ..

J:-'

i:0

100

(e)

Equivalence Ratio0.4 0.6

0.8

0 1 .0

0

0.2

Fig. 44. (a) Abiabatic reaction temperature forbiomass ofatomic composition CH1.400.6 reacting with oxygen and air, plotted against the equivalence ratio, $, the ratio of oxygen to that required for complete combustion (b) Equilibrium gas composition for reaction with air (e) Energy in solid and gas (d) Energy per volume of gas (Source: Reed 1981, Figs. S-2 - S5)

(d)

Equivalence Ratio0.4 0.6

0.8

1.0

26


(up to one mile) in pipelines. It is also called synthesis gas. since it can be used as a feedstock for the chemi cal synthesis of methanol. ammonia. methane. and gasoline. The oxygen must be either purchased or produced on-site. making it economically prudent only in larger installations. It has been reported that pipeline distribution of low-energy gas is also economically practical for distances up to one mile if the air used for gasification is compressed. rather than compressing the larger volume of producer gas (McGowan 1984). There are many types of direct gasifiers. each with its special virtues and defects. They will be discussed in Chapter 5 .

Fuel hopper-'I I

I

I

I I

: := :H =l=;2=' '9 Reduction I I..-"' ...,

4.4 Principles of Operation o f Direct Gasifiers 4.4.1 IntroductionSince volatile organic molecules make up ap proximately 80% of the products from biomass pyrolysis (Diebold 1985b). the principal task in biomass (but not coal) gasification is to convert this condensible volatile matter to permanent gases. A secondary task is to convert the resulting charcoal also to gas. The most important types of fixed-bed gasifiers for this task are the updraft and downdraft gasifiers of Fig. 4-5. These gasifiers will be discussed in greater detail in Chapter 5. but a brief introduction here will facilitate understanding of the fundamental principles involved. The terms "updraft gasifier" and " downdraft gasifier" may seem like trivial mechanical descriptions of gas flow patterns. In practice. however. updraft biomass gasifiers can tolerate high moisture feeds and thus have some advantages for producing gas for combustion in a burner. However. updraft gasifiers produce 5 % to 20% volatile tar-oils and so are unsuitable for opera tion of engines. Downdraft gasifiers produce typically less than 1 % tar-oils and so are used widely for engine operation. The reasons for this difference are given below.

:

py

ro=fAsh

Gas -C + CO, 2CO C + H,O CO + H,= = =

I f:: Combustion -1 C + 0,---

(a)

'-'-:Ai:r "' t -- t _

CO,

Fuel hopper

_ _

..., Gas

Pyrolysis C + 0, CO,=

Reduction C CO, 2CO C + H,O CO + H, -""'Ash L-J+= =

4.4.2 Operation of the Updraft GasifierThe updraft gasifier is shown schematically in Fig. 4-5(a). Biomass enters through an air seal (lock hopper) at the top and travels downward into a rising stream of hot gas. In the pyrolysis section. the hot gas pyrolyzes the biomass to tar-oil. charcoal. and some gases. In the reduction zone the charcoal thus formed reacts with rising COz and HzO to make CO and Hz. Finally. below the reduction zone incoming air burns the charcoal to produce COz and heat (Desrosiers 1982; Reed 1985b). Note that the combustion to COz is exothermic. and the heat produced in the gas here is

(b) Fig. 45. Schematic diagram of (a) updraft and (b) downdraft gasifier showing reactions occurring in each zone (Source: Reed 1981, Figs. 86. 87)

absorbed in the endothermic reduction and pyrolysis reactions above. Depending upon the pyrolysis conditions in a gasifier. one can generate a wide range of vapors (wood oil and wood tar) in the hot gas. If the pyrolysis products are to be burned immediately for heat in a boiler or for drying (close-coupled operation). then the presence of condensible vapors in the gas is of little importance. In


27

fact, the condensible tars represent a high-energy fuel and greatly enhance the energy obtained from each unit volume of biomass. If the volatile materials are condensed, they produce tars and oils known commonly as creosote. These materials collect in the chimneys of airtight wood stoves,the piping of gasifiers, and the valves of engines. Most of the companies advertising and selling updraft gasifiers at a 1979 conference no longer produce them (Reed 1979). If the gas is to be conveyed over a distance in a pipeline, burned in any form of engine, or used as a chemical feedstock, the condensing tars will plug pipes some times in only a few minutes. In these cases, it is neces sary to use a mode of gasification that succeeds in converting the tars to gas. This can be accomplished either by cracking (secondary pyrolysis) or by partial oxidation in flaming pyrolysis.

Although flaming pyrolysis is a new concept in ex plaining biomass gasification, partial oxidation of small and large hydrocarbon molecules to CO and Hz is a standard industrial process. Texaco has used an oxygen gasifier to oxidize hydrocarbons to CO and Hz, as in the following reaction for a typical oil:C lOHzz + 5 0z

10 CO + 11 Hz

(4-5)

The resulting gas, called synthesis gas, can be used to manufacture methanol, hydrogen, or anunonia. There is some interest in using the Texaco system to gasify biomass (Stevenson 1982).

4.4.4 Factors ContrOlling Stability of Gasifier OperationGasifer operating temperature is a function of the amount of oxygen fed to the gasifier (Fig. 4-4(a)). The temperature response, however, changes abruptly at an equivalence ratio (ER) of approximately 0.25. This change point, or knee, occurs for temperatures of 600' to 800'C (900-1100 K), depending on oxygen source. Gasifier pyrolysis produces oils and tars that are stable for periods of 1 second or more at temperatures below 600C. Since updraft gasifiers operate below an ER of 0.25 (temperatures less than 600'C), considerable quantities of tars are emitted with the product gas. In the gasifier of Fig. 4-5(b), air is injected at the inter face between the incoming biomass and the char. If too much char is produced, the air consumes the excess char rather than biomass; if the char is consumed too fast, more biomass is consumed. Thus, the Imbert gasifier is self regulating. At SERI we have built the oxygen gasifier shown in Fig. 5-12. We operate this with a fixed flow of oxygen and add biomass faster or slower to maintain a fixed bed level. In the Buck Rogers gasifier of Fig. 5-11, a fraction of air is introduced through the rotating nozzles and maintains the zone at that level (Walawender 1985). Some gasifiers operate at lower values of on purpose by augering charcoal out of the char zone in order to produce charcoal-a valuable byproduct-and to yield the higher gas heating value shown at low in Fig. 4-4(d). Such operation is not a true gasification but might be called "gas/charification." In entrained or fluidized bed operation, the ratio of biomass to oxygen can be varied independently. In this case must be set, typically by fixing oxidant flow and varying fuel flow to maintain a constant temperature.

4.4.3 Operation of the Downdraft GasifierDowndraft gasifiers have been very successful for operating engines because of the low tar content. Most of the work reported in this book was performed on downdraft systems, and they will be the principal gasifier discussed in the balance of this book. In the downdraft gasifier of Fig. 4-5 (b), air contacts the pyrolyzing biomass before it contacts the char and sup ports a flame similar to the flame that is generated by the match in Fig. 4-2. As in the case of the match, the heat from the burning volatiles maintains the pyrolysis. When this phenomenon occurs within a gasifier, the limited air supply in the gasifier is rapidly consumed, so that the flame gets richer as pyrolysis proceeds. At the end of the pyrolysis zone, the gases consist mostly of about equal parts of COz, HzO, CO, and Hz. We call this flame in a limited air supply "flaming pyrolysis," thus distinguishing it from open wood flames with un limited access to air (Reed 1983a). Flaming pyrolysis produces most of the combustible gases generated during downdraft gasification and simultaneously con sumes 99% of the tars. It is the principal mechanism for gas generation in downdraft gasifiers. If the formula for biomass oil is taken as approximate ly CH1 .20o.5' then partial combustion of these vapors can be represented approximately by the reaction:CH1.2 0o.5 + 0.6 0z

0.5 CO + 0.5 COz

+

0.4 Hz + 0.2 HzO

(4-4)

4.5 Charcoal GasificationThe manufacture of charcoal for use as a synthetic fuel dates back at least 10,000 years and is closely as sociated with the development of our civilization. Today, charcoal is used as the prime source of heat for cooking in less developed countries and also is used for the reduction of many ores in smelting processes.

(The exact 0z-to-vapor-ratio will depend on the exact vapor composition and gasifier conditions.) Downdraft gasifiers usually produce vapors that are less than 1 % condensible oilltar, the reason behind the almost ex clusive use of downdraft gasifiers as an energy source for operating engines.28


The charcoal yield from a biomass feedstock is highly dependent on the rate of heating and the size of the biomass particles. Industrial charcoal manufacture uses very slow heating rates to achieve charcoal yields of more than 30% of the initial dry weight of the biomass. The intermediate heating rates used in proximate analysis usually produce charcoal yields of 15% to 20% . The very rapid heating rates encountered when small biomass particles are gasified and com busted realize charcoal yields of less than 15% of the initial dry weight of the biomass; larger size feedstocks produce 15% to 25% charcoal. During updraft or downdraft gasification, 10% to 20% of the biomass will remain as charcoal after pyrolysis is complete. In an updraft gasifier, air entering at the grate initially burns this char to liberate heat and CO2 according to the reaction: (4-6) Almost immediately, or even simultaneously, the CO2 and any H20 present in the gasifier react with the char to produce the fuel gases CO and H2 according to the following reactions: C + CO2 C + H20->

their cooling effect helps to keep the gas temperature from rising 'above this temperature. Below 800"C, the reactions become sluggish and very little product forms. We have modeled the reactions of downdraft char gasification using known kinetic values and find that the temperatures measured in char gasification correspond to those observed in the gasifier (Reed 1983a; Reed 1984). We refer to the process observed in an actual bed of char as adiabatic (no heat input) chargasification.

The CO and H2 formed in the hot char zone can react below 900"C to form methane according to the reaction: CO + 3 H2->

CH4

+

H20

(4-9)

This reaction proceeds slowly unless there is a catalyst present; however, it is quite exothermic and can supply heat if suitably catalyzed. Concurrent with the emergence of biomass as an im portant energy source, it was natural that coal gasifica tion interpretations would be carried over to explain biomass gasification. Even today, most articles on biomass gasification use only Eqs. (4-7) and (4-8) to ex plain biomass gasification and ignore Eq. (4-4), even though Eq. (4-4) applies to the 80% biomass volatiles. Biomass pyrolysis produces only 10% to 20% char coal, and the charcoal is very reactive. Therefore, this cannot be the primary explanation for the conversion of biomass to gas.

2 CO

(4-7) (4-8)

->

CO+ H2

The first reaction is called the Boudouard reaction, and the second is called the water-gas reaction. They have been studied extensively for the last 100 years in con nection with coal and biomass gasification, since the principal product of coal pyrolysis is coke (carbon). The rate of the reaction has been studied by measuring the rate of disappearance of carbon, coal, or charcoal while passing H20 or CO2 over the solid (Nandi 1985; Edrich 1985). Both of these reactions require heat (Le., they are en dothermic reactions) and therefore cool the gas about 25"C for every 1 % of CO2 that reacts. These reactions occur very rapidly at temperatures over 900"C, and

4.6 SummaryIn summary, the task of a gasifier is threefold:

to pyrolyze biomass to produce volatile matter, gas, and carbon to convert the volatile matter to the permanent gases, CO, H2, and CH4 to convert the carbon to CO and H2.

These tasks are accomplished by partial oxidation or pyrolysis in various types of gasifiers.


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Chapter 5 Gasifier Designs

5.1 IntroductionMany different designs of gasifiers have been built and are described in the extensive literature on this subject (see especially Gengas 1950; Skov 1974; Foley 1983; Kjellstrom 1983, 1985; Kaupp 1984a; NAS 1983). Much of this material has been collected by A. Kaupp of the University of California at Davis. (Copies of these papers are also at SERI and the German Appropriate Technology Exchange [GATE] in Eschborn, West Ger many.) Anyone interested in design modification and improvement would be well-advised to become ac quainted with this material before repeating tried and tested techniques. However, many of the documented design variations are minOT. We believe that future improvements to gasifiers will be based on a better understanding of the basic proces ses, combined with improved measurements of gasifier behavior and better regulation of fuel properties. Work is under way at various private and public centers to increase our understanding of the gasification process. Consequently, gasifier design is in a state of flux. This makes it difficult to organize a "handbook of gasifier design" without having it out of date before the ink is dry. To avoid this problem, we will first describe the con struction and operation of a number of historical gasifiers described in the literature to aid in under standing various tradeoffs still under development. The reader must remember that the choice of gasifier is dictated both by the fuels that will be used and the use to which the gas will be put. We will then describe some gasifiers currently under development.

Fig. 5-1. Diagram of downdraft gasification (Source: Skov 1974, Fig. 14. 1974. Used with permission of Biomass Energy Foundation, Inc.)

Spring safety lid Air seal

Un fuel Gas cooling ---oL! ""J

Cyclone

5.2 Basic Gasifier TypesFixed bed (sometimes called moving bed) gasifiers use

Flaming

a bed of solid fuel particles through which air and gas pass either up or down. They are the simplest type of gasifiers and are the only ones suitable for small-scale application.

Engine suction

Air inlet

' " a.a

The downdraft gasifier (Figs. 4-5(b), 5-1, and 5-2) was developed to convert high volatile fuels (wood, biomass) to low tar gas and therefore has proven to be the most successful design for power generation. We concern ourselves primarily with several forms of downdraft gasifiers in this chapter. The updraft gasifier (Figs. 4-5(a), 5-3, and 5-4) is wide ly used for coal gasification and nonvolatile fuels such as charcoal. However, the high rate of tar production

shaker

Fig. 5-2. Imbert (nozzle and constricted hearth) gasifier (Source: Gengas 1950, Fig. 75)

30


5.3 Charcoal GasifiersGasdeveloped for vehicle operation. They are suitable only for low-tar fuels such as charcoal and coke. Figure 5-4 shows an updraft charcoal gasifier that was used in the early part of World War II. Air enters the updraft gasifier from below the grate and flows upward through the bed to produce a combustible gas (Kaupp 1984a). High temperatures at the air inlet can easily cause slagging or destruction of the grate, and often some steam or CO2 is added to the inlet air to moderate the grate tempera ture. Charcoal updraft gasifiers are characterized by comparatively long starting times and poor response because of the large thermal mass of the hearth and fuel zone. Charcoal manufacture is relatively simple and is car ried on in most countries. However, it requires tight controls on manufacturing conditions to produce a charcoal low in volatile content that is suitable for use in charcoal gasifiers.Updraft charcoal gasifiers were the first to b e

Hearth Zone

Ash ZoneFig. 5-3. Diagram of updraft gasification (Source: Skov 1974 Fig. 9. 1974. Used with permission of Biomass Energy Foundation, Inc.)

(5%-20%) (Desrosiers 1982) makes them impractical for high volatile fuels where a clean gas is required.Fluidized beds are favored by many designers for

5.4 Charcoal versus Biomass FuelsHigh-grade charcoal is an attractive fuel for gasifiers be cause producer gas from charcoal, which contains very little tar and condensate, is the simplest gas to clean. Charcoal gasifiers were restricted over much of Europe during the later years of World War II because charcoal

gasi fiers producing m OTe than 40 GJ(th)/h * [40 MBtu(th)/hl and for gasifiers using smaller particle feedstock sizes. In a fluidized bed, air rises through a grate at high enough velocity to levitate the particles above the grate, thus forming a "fluidized bed." Above the bed itself the vessel increases in diameter, lowering the gas velocity and causing particles to recirculate within the bed itself. The recirculation results in high heat and mass transfer between particle and gas stream.Suspended particle gasifiers move a suspension of biomass particles through a hot furnace, causing pyrolysis, combustion, and reduction to give producer gas. Neither fluidized bed nor suspended particle gasifiers have been developed for small-scale engine use.

Water Hopper Blower

We have already mentioned that gasifier designs will differ for different feedstocks, and special gasifiers have been developed to handle specific forms of biomass feedstocks, such as municipal solid wastes (MSW) and rice hulls. The manner in which ash is removed determines whether the gasifier is classified as either a dry ash (ash is removed as a powder) or slagging (ash is removed as a molten slag) gasifier. Slagging updraft gasifiers for biomass and coal have been operated at only a very large scale. *The units Hth) and Btu(th) refer to the thennal or chemical energy produced. This can be converted to electricity with an efficiency of 1 0% to 40%. so the electrical energy content (J or Btu) will be propor tionally lower.

Fire

Outlet -

Rg. 5-4. Updraft coke andcharcoalgasifier, early World W arII (Source: Kaupp 1984a, Fig. 27)

Gasifier Designs

31

manufacture wastes half of the energy in the wood (Gengas 1950). On the other hand. Australia worked al most exclusively with charcoal during this period be cause of that country's large forest acreage and small number of vehicles. Nevertheless, the simplicity of charcoal gasification has attracted many investigators, and more than 2000 charcoal systems have been manufactured in the Philippines. A large number are not currently working (Kadyszewski 1986).

hearth zone with unpyrolyzed biomass, leading to momentarily high rates of tar production. The fuel size also is very important for proper operation. Cross draft gasifiers have the fastest response time and the smal lest thermal mass of any gas producers because there is a minimum inventory of hot charcoal. In one design, a downdraft gasifier could be operated in a cross draft scheme during startup in order to minimize the startup time (Kaupp 1984a).

5.6 The Updraft GasifierThe updraft gasifier has been the principal gasifier used for coal for 150 years, and there are dozens in opera tion around the world. In fact, World War II-type Lurgi gasifiers now produce a large share of the gasoline used in South Africa by oxygen gasification followed by Fischer-Tropsch catalytic conversion of the gas to gasoline. The geometry of the updraft gasifier is shown in Figs. 4-5(a), 5-3, and 5-4. During operation, biomass is fed into the top while air and steam are fed through a grate, which often is covered with ash. The grate is at the base of the gasifier, and the air and steam react there with charcoal from the biomass to produce very hot COz and HzO. In turn, the COz and HzO react endothermically with the char to form CO and Hz according to Eqs. (4-6) through (4-8). The temperatures at the grate must be limited by adding either steam or recycled exhaust gas to prevent damage to the grate and slagging from the high temperatures generated when carbon reacts with the air. The ascending, hot, reducing gases pyrolyze the incom ing biomass and cool down in the process. Usually, 5% to 20% o f the tars and oils are produced at tempera tures too low for significant cracking and are carried out in the gas stream (Desrosiers 1982). The remaining heat dries the incoming wet biomass, so that almost none of the energy is lost as sensible heat in the gas. The updraft gasifier throughput is limited to about z 10 GJ/h-m (l06 Btu/h-ftZ) either by bed stability or by incipient fluidization, slagging, and overheating. Large updraft gasifiers are sometimes operated in the slagging mode, in which all the ash is melted on a hearth. This is particularly useful for high-ash fuels such as MSW; both the Purox and Andco Torax processes operate in the slagging mode (Masuda 1980; Davidson 1978). Slagging updraft gasifiers have both a slow response time and a long startup period because of the large thermal mass involved.

5.5 The Crossdraft GasifierThe cross draft gasifier shown in Fig. 5-5 is the simplest and lightest gasifier. Air enters at high velocity through a single nozzle, induces substantial circulation, and flows across the bed of fuel and char. This produces very high temperatures in a very small volume and results in production of a low-tar gas, permitting rapid adjustroent to engine load changes. The fuel and ash serve as insulation for the walls of the gasifier, permit ting mild-steel construction for all parts except the noz zles and grates, which may require refractory alloys or some cooling. Air-cooled or water-cooled nozzles are often required. The high temperatures reached require a low-ash fuel to prevent slagging (Kaupp 1984a). The cross draft gasifier is generally considered suitable only for low-tar fuels. Some success has been observed with unpyrolyzed biomass, but the nozzle-to-grate spacing is critical (Das 1986). Unscreened fuels that do not feed into the gasifier freely are prone to bridging and channeling, and the collapse of bridges fills the

Distillation zone Air Hearth zone

Reduction zone_ _

Gas

5.7 The Imbert Downdraft GasifierAsh pitFig. 5-5. Diagram of crossdraft gasification (Source: Skov 1974, Fig. 18. 1974. Used with permission of Biomass Energy Foundation,

5.7.1 IntroductionThe nozzle (tuyere) and constricted hearth downdraft gasifier shown in Figs. 4-5(b), 5-4, and 5-5 is sometimes

32


called the "Imbert" gasifier (after its entreprenurial in ventor, Jacques Imbert) although it was produced by dozens of companies under other names during World War II. Approximately one million of these gasifiers were mass produced during World War II, at a cost of about $1000 u.s. (1983) each. It is important to realize that the cost of producing such a unit today would depend primarily on the degree to which it could be mass produced since none of the components are inherently expensive. Air gasifiers can be operated either by forcing air through the fuel (pressurized) or by drawing the air through the fuel (suction). In practice, gasifiers that fuel engines generally use the suction of the engine to move air through the gasifier and cleanup train, and these are called "suction gasifiers." We will describe only suc tion gasifiers here; however, only minor modifications are required to build pressurized gasifiers. (See Chap ter 8, which deals with the topics of blowers, fans, ejectors, and compressors). A large number of descriptive articles on gasifiers ap peared during World War II, but no detailed drawings have been located from that period. Fortunately, for mulas for determining critical dimensions are given in a number of the older references (Gengas 1950; Schliipfer 1937). Renewed interest in biomass gasification has manifested itself in the fact that a number of in dividuals and groups have built modern versions of the Imbert gasifier. Plans and manuals for constructing some of these designs are available from several groups (Mother 1982; Skov 1974; Nunnikhoven 1984; Rissler 1984). Some of these gasifiers have been attached to cars and trucks that have succeeded in traversing the country on several occasions. In particular, Mother Earth News and its subsidiary, Experimental Vehicle News, have performed extensive tests on gasifiers and have published informative articles and plans with photographs of fabrication steps. The plans are suffi ciently detailed so that a skilled welder can fabricate a gasifier for a relatively small expense. In 1978, a number oftests were performed under a SERI contract on a 75-hp "Hesselman" (Imbert-type) downdraft gasifier. This gasifier was built in Sweden at the end of World War II and was imported to this country by Professor Bailie of the University of West Virginia. Professor Bailie used the gasifier in tests during which the gasifier operated on wood, wood pel lets, and oxygen (Bailie 1979). Subsequently, the gasifier was sent to SERI in Colorado for further testing with a 15-kW Onan electric generator. More recently, the gasifier has been used to gasify peat by Professor Goldhammer of Lowell University. The gasifier is now being used by Syngas Systems, Inc., to generate producer gas to test gas cleanup systems for use with its 750-kW power generator. Although much ofthe test

ing was qualitative in nature, the authors have had con siderable experience in running this interesting tech nological antique.

5.7.2 Description of the Downdraft (Imbert) GasifierReferring to Figs. 5-1 and 5-2, the upper cylindrical part of the inner chamber is simply a magazine for the wood chips or other biomass fuel. During operation, this chamber is filled every few hours as required. The spring-loaded cover is opened to charge the gasifier, and then it is closed during gasifier operation. The spring permits the cover to pop open to relieve pres sure in the case of a gas explosion, thus functioning as a safety valve. About one-third of the way up from the bottom, there is a set of radially directed air nozzles that permit air to be drawn into the chips as they move down to be gasified. Typically, there are an odd number of nozzles so that the hot gases from one nozzle do not impinge on the opposite nozzle. The nozzles are attached to a distribution manifold that in turn is attached to the outer surface of the inner can. This manifold is con nected through the outer can to a large air-entry port. One air nozzle is in line with this port, allowing the operator to ignite the charcoal bed through this nozzle. During operation, the incoming air burns and pyrolyzes some of the wood, most of the tars and oils, and some of the charcoal that fills the gasifier below the nozzles. Most of the mass of biomass is converted to gas within this flaming combustion zone since biomass contains more than 80% volatile matter (Reed 1983a). The gasifier is in many ways self-adjusting. If there is insufficient charcoal at the air nozzles, more wood is burned and pyrolyzed to make more charcoal. If too much char forms during high-load conditions, then the char level rises above the nozzles so that incoming air burns the char to reduce the char level. Thus, the reaction zone is maintained at the nozzles. Below the air nozzle zone lies the gas-reduction zone, usually consisting of a classical Imbert hearth (Fig. 5-2) or in later years, of the "V" hearth (Fig. 5-6). Most recently, the flat-plate hearth constriction (Fig. 5-7) has been introduced. The latter two hearth designs accumulate a layer of retained ash to form a high-quality, self-repairing insulation. Improved insulation in the hearth results in lower tar production and a higher efficiency over a wider range of operating conditions. After the combustion/pyrolysis of wood and hot char at the nozzle level (see below), the resulting hot com bustion gases (COz and HzO) pass into this hot char where they are partially reduced to the fuel gases CO and Hz according to Eqs. (4-7) and (4-8). This procedure

Gasifier Designs

33

Cast-iron constriction (ing

tions. Usually, wood contains less than 1 % ash. However, as the charcoal is consumed, it eventually collapses to form a powdered char-ash that may repre sent 2% to 10% of the total biomass, in turn contain ing 10% to 50% ash. Ash contents depend on the char content of the wood and the degree of agitation. The greater the degree of char reduction, the smaller the resulting particles and the higher the ash, as shown in Fig. 3-3. The downdraft gasifier startup and response time is intermediate between the fast cross draft gasifier and the slow updraft gasifier. The Imbert gasifier requires a low-moisture 20% moisture) and uniformly blocky fuel in order to allow easy gravity feeding through the constricted hearth. Twigs, sticks, and bark shreds must be completely removed. The reduction in area at the hearth and the protruding nozzles present hazards at which the pas sage of the fuel can be restricted, thus causing bridging and channeling followed by high tar output, as un pyrolyzed biomass falls into the reaction zone. The

I ron-plate hearth mantle Inside insulation by ashes Cast-iron v-hearth, easily removable

Fig. 5-6. V-hearth Imbertgasifier (Source: Gengas 1950, Fig. 74)

results in a marked cooling of the gas, as sensible-gas heat is converted into chemical energy. This removes most of the charcoal and improves the quality of the gas. Eventually, the charcoal is " dissolved" by these gases and disintegrates to smaller chunks and a fine powder that either is swept out with the gases to the cyclone separator or falls through the grate. Tars that have escaped combustion at the nozzle may crack fur ther in the hot char although tar cracking is now thought to occur only above about 850C (Kaupp 1984b; Diebold 1985). The spaces between the nozzles (shown in Fig. 5-8) allow some unpyrolyzed biomass to pass through. The hearth constriction then causes all gases to pass through the hot zone at the constriction, thus giving maximum mixing and minimum heat loss. The highest temperatures are reached in this section so the hearth constriction should be replaceable. If tarry gas is produced from this type of gasifier, common practice is to reduce the hearth constriction area until a low-tar gas is produced. However, one should remember that hearth dimensions also play a role in the gas production rate (see below). The fine char-ash dust can eventually clog the charcoal bed and will reduce the gas flow unless the dust is removed. The charcoal is supported by a movable grate that can be shaken at intervals. Ash builds up below' the grate and can be removed during cleaning opera

Fig. 5-7. Flat-plate hearth constriction (Source: Gengas 1950, Fig. 76)

34


units. This term enables one to compare the perfor mance of a wide variety of gasifiers on a common basis. The maximum specific hearth loads for a number of gasifiers are shown in Table 5-1. The table was calcu lated from data available on gasifiers that have been thoroughly tested and lists the maximum superficial velocity and heating load reported. Note that in European literature, hearth load is reported in gas volume units; in the United States, it is reported in energy units. In Generator Gas (Gengas 1950) a maximum heartb load (Bhmax) value for an Imbert-style gasifier is about 0.9 Nm3/h-cm2 . In other words, 0.9 m3 of gas is produced for each square centimeter of cross-sectional area at the constriction. This corresponds to a superficial gas velocity Vs of 2 . 5 m/s (8.2 ft/s) calculated at NTP* from the throat diameter and ignoring tbe presence of fuel. This corresponds to a specific gas production rate of 9000 m3 of gas per square meter of cross-sectional area per hour (29,500 scf/ft2-h). If the gas has a (typical) energy content of 6.1 MJ/Nm3 (150 Btu/scf), this results in a specific energy rate of 54.8 GJ/m2-h (4.4 MBtu/ft2-h). The diameter of the pyrolysis zone at the air nozzles is typically about twice tbat at the tbroat, and Table 5-1 shows the hearth load on this basis also. This puts the hearth load for the Imbert type gasifier on a comparable basis to the stratified downdraft gasifier. Knowledge of maximum hearth load permits one to calculate tbe size of hearth needed for various engine or burner sizes. Dimensions for a variety of Imbert-type gasifiers are shown in Tables 5-2 and 5-3. The maximum hearth load is limited by many factors, such as the mechanical integrity of the char bed struc ture witbin the gasifier, degree of agitation, and the time available for conversion. High velocities can dis turb the char and fuel bed, causing instability. If char fragments become dislodged and airborne, tbey may plug tbe bed or form channels. Therefore, a little agita tion can effectively increase tbe maximum specific heartb load. The heating value of producer gas varies witb flow rate, as shown in Fig. 7-20. Notice that the maximum ef ficiency for rice hulls occurs at twice the flow rate tbat produces the maximum heating value from rice hulls. This occurs because the combination oflower tempera tures and low flow rate favors metbane and tar produc tion. Altbough the change in efficiency is small, tbe benefit of reducing tar production is substantial. Closely

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